1. Field of the Invention
The present invention relates to a semiconductor device having a power cutoff transistor, adapted to control the supply and interruption of a source or reference voltage to a logic circuit, formed in the same semiconductor substrate as a logic transistor of the logic circuit.
2. Description of the Related Art
Recent years have seen an increase in transistor leak current in an OFF state as a result of smaller minimum dimensions required by the wafer process of the semiconductor device manufacturing (hereinafter simply referred to as the “process”). This has pushed up the percentage of useless power consumption caused by leak current in the overall power consumption of the semiconductor integrated circuit (chip). Today, this percentage has grown to a level which is no longer negligible.
Power gating is used as a way of reducing this useless power consumption. Power gating is also known as a circuit technique called MTCMOS (multi-threshold complementary metal oxide semiconductor).
Power gating is used by inserting a transistor having a high threshold voltage (power cutoff transistor) in series with a path adapted to supply a source current to a logic circuit. The power cutoff transistor is turned off during a period of time when the logic circuit can be inactivated. This cuts off the source current supply path, thus ensuring reduced leak current.
However, the above requires two different types of transistors, namely, a logic transistor of the logic circuit and a power cutoff transistor with a high threshold voltage, to be formed on the same semiconductor substrate, thus resulting in higher process cost.
To keep down the cost to the extent possible, the affinity (commonality) in process between the logic and power cutoff transistors has to be enhanced.
To achieve this, the same structures and manufacturing condition parameters have to be used for the different processes to the extent possible, that is, a higher percentage of the two types of transistors has to be manufactured in a single step. For example, the threshold voltage is changed only by controlling the channel concentration. Other parameters of the two transistors remain the same. Among the parameters which are identical between the two transistors are the type and concentration of impurities introduced into the source and drain regions, the material and thickness of the gate insulating film, and the material and thickness of the gate electrode.